Computing devices, such as notebook computers, personal data assistants (PDAs), and mobile handsets, have user interface devices, which are also known as human interface device (HID). One user interface device that has become more common is a touch-sensor pad. A basic notebook touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse x/y movement by using two defined axes which contain a collection of sensor elements that detect the position of a conductive object, such as finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a cursor, or selecting an item on a display. These touch-sensor pads can include multi-dimensional sensor arrays. The sensor array may be one dimensional, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.
FIG. 1A illustrates a conventional touch-sensor pad 100. The touch-sensor pad 100 includes a sensing surface 105 on which a conductive object may be used to position a cursor in the x- and y-axes. Touch-sensor pad 100 may also include buttons such as left and right buttons 110 and 115, respectively. These buttons 110 and 115 are typically mechanical buttons, and operate much like left and right buttons on a mouse. These buttons 110 and 115 permit a user to select items on a display or send other commands to the computing device.
FIG. 1B illustrates another aspect of the conventional touch-sensor pad 100. The touch-sensor pad 100 includes a plurality (e.g., twelve) of metal strips 120, which are also referred to as sensor elements. The sensor elements 120 are coupled to the processing device 125, which includes a plurality of capacitance sensors 130. The capacitance sensors 130 are also referred to as capsensors. Typically, each sensor element 120 is coupled to a corresponding capsensor 130 via an independent pin 135 (i.e., metal pin extending from the chip package of the processing device 125). Thus, the illustrated touch-sensor pad 100 has twelve pins 135 to independently couple each of the twelve sensor elements 120 to the corresponding capsensors 130 within the processing device 125. This one-to-one correspondence between sensor elements 120 and capsensors 130 limits the number of sensor elements 120 and, therefore, the size of a touch-sensor pad 120. Additionally, the functionality of the processing device 125 is limited by the number of pins 135 used for the capsensors 120 because the processing device 125 is not able to electrically connect to other components of the touch-sensor pad 100.
In general, the sensor elements 120 can be used to determine the location or position of a conductive object 140 such as a finger or stylus on the touch-sensor pad 100. For ease of discussion and illustration, the depicted touch-sensor pad 100 includes sensor elements 120 to detect the location and directional movement of the conductive object 140 along a single axis, for example, left-right. One type of touch-sensor pad 100 that detects only directional movement, but not necessarily the location, of the conductive object 140 is referred to as a directional slider. Other touch-sensor pads 100 detect movement in multiple directions such as left-right and up-down.
In order to determine the location and/or directional movement of the conductive object 140 relative to the sensor elements 120, the processing device 125 implements linear search algorithms on the sensor elements 120. With a linear search algorithm, capacitance variations of the sensor elements 120 are detected one-by-one in a linear fashion at the corresponding capsensors 130. By comparing the detected capacitance variation of a sensor element 120 with a baseline capacitance and the capacitance variations on neighboring sensor elements 120, the position of the conductive object 140 (e.g., x coordinate) is determined. For example, the processing device 125 may first detect the capacitance variation on the first sensor element 120, then on the second sensor element 120, and so on from left to right across several sensor elements 120. The processing device 125 implements this linear search algorithm by sequentially sampling the capsensors 130. If the conductive object 140 is on the first sensor element 120, then the processing device 125 only takes one cycle to sample the first capsensor 130. If the conductive object 140 is on the n-th sensor element 120, then the processing device 125 takes as many as n cycles to sample the first through the n-th capsensor 130. Accordingly, the processing device 125 takes, on average, (n+1)/2 cycles to locate the contacting point of the conductive object 140 using this linear search algorithm.
Accordingly, the one-to-one correspondence between the sensor elements 120, capsensors 130, and pins 135 limits the functionality of the processing device 125 and impacts the number of cycles used to detect the position or movement of the conductive object 140.